In the past, when excavating ground at underground construction sites or performing ground improvement, groundwater is pumped to lower the groundwater level. Thus, by pumping groundwater to lower the groundwater level, it is possible to reduce a water content in soils of ground at a desired location. As a result, the excavating operation or treatments for the excavated soils can be facilitated.
As a groundwater pumping apparatus, a deep-well apparatus is known from the past. For example, in a conventional deep-well apparatus shown in FIG. 11, a well 40 having a water-passing portion 41 (strainer), through which groundwater comes inside, is put in a boring 43 formed in ground 42. In addition, a pump 46 is placed in the well 40. A clearance between an inner wall of the boring 43 and an outer surface of the well 40 is filled with a filter material 45 such as coarse sand or pea gravel. Groundwater is drawn from the water-passing portion 41 in the well 40 by the aid of a hydraulic head drop between a water level in the well 40 and the natural water level, and then the collected groundwater is pumped by the pump 46.
However, since the hydraulic head drop between the water level in the well 40 and the natural water level is used to collect groundwater, the groundwater collecting capacity depends on gravity difference. For this reason, groundwater can not be collected under a condition of effectively lowering the groundwater level.
In addition, a vacuum deep-well apparatus is known, which has the capability of effectively lowering the groundwater level by placing a vacuum unit in the deep-well apparatus, and reducing a pressure in the well, is known. However, in this vacuum deep-well apparatus, there is a problem that when the groundwater level reaches a position lower than the top end of the water-passing portion, air is drawn together with groundwater in the well through the water-passing portion, so that the vacuum effect of the vacuum unit sharply deteriorates. Thus, it still has plenty of room for improvement from the point of stably providing the groundwater collecting/pumping operation.
On the other hand, as shown in FIG. 12, a groundwater pumping apparatus disclosed in Japanese Patent Early Publication No. 2000-27170 has a strainer device 48 at a lower end of a casing tube 47 buried in ground 42. This strainer device 48 is formed with a strainer tube 49 attached to the lower end of the casing tube 47, sand accumulator 50 formed at a lower end of the strainer tube 49, and an inner tube 51 attached to the strainer tube 49 in a concentric manner with the casing tube 47. The strainer tube 49 is attached such that an outer surface of the casing tube 47 is substantially flush with the outer surface of the strainer tube 49. A clearance 52 is made between the strainer tube 49 and the inner tube 51, and closed at its upper end by a blockage plate 53 placed at the lower end of the casing tube 47. A groundwater inlet 54 is formed in the inner tube 51 at a position lower than the top end of a water-passing portion 55 of the strainer tube 49. In addition, a pump 56 for pumping groundwater is placed in the inner tube 51. The top end of the casing tube 47 is closed by a cover 58. In FIG. 12, the numeral 57 designates a vacuum unit for reducing a pressure in the casing tube 47 connecting to the inner tube 51.
In the case of using this pumping apparatus, when the pressure in the casing tube 47 connecting to the inner tube 51 is reduced by use of the vacuum unit and the hydraulic head drop, groundwater is drawn from the water-passing portion 55 of the strainer tube 49 in the inner tube 51 through the clearance 52 and the groundwater inlet 54, and collected in the inner tube 51. The collected groundwater in the inner tube 51 is pumped by the pump 56. Even when the groundwater level is lower than the top end of the water-passing portion 55, no intrusion of air into the inner tube 51 is caused under a condition that the groundwater level is higher than the position of the groundwater inlet 54. Therefore, it is possible to prevent the deterioration of the vacuum effect by the air intrusion, which occurs in the vacuum deep-well apparatus described above.
However, in this groundwater pumping apparatus, it is necessary to place the strainer device at the lower end of the casing tube 47. Therefore, when a deep-well apparatus having poor groundwater collecting capability has already existed in ground 1, the groundwater collecting/pumping operation by use of the already-existing apparatus is stopped, and the pumping apparatus of JP 2000-27170 must be newly formed in ground 1 to restart the water collecting/pumping operation. Thus, there is a problem that the existing deep-well apparatus comes to naught, and constructions for a groundwater pumping apparatus having another structure become necessary.
Moreover, since the groundwater pumping apparatus of JP 2000-27170 has the structure that the strainer device 48 composed of the inner tube 51, the clearance 53 and the strainer tube 49 is placed under the casing tube 47, groundwater can be collected only through the strainer device 48 placed under the casing tube 47. Therefore, when groundwater is present in the vicinity of an upper part of the casing tube 47 due to stratum structure, it may not be effectively collected. In addition, in this pumping apparatus, when the groundwater level lowers to reach the groundwater inlet 54 of the inner tube 51, air is sharply drawn in the inner tube 51 in place of groundwater, so that the vacuum effect sharply deteriorates. For this reason, the pump 56 must be stopped until the groundwater level raises again. Subsequently, when the groundwater level is recovered, the pressure in the inner tube 51 is reduced to a required value by the vacuum unit 57 and the groundwater collecting/pumping operation is restarted by use of the pump 56. Thus, there is a case that a stable groundwater collecting/pumping operation can not be continuously carried out.